Gas chromatographs are widely used to measure quantities of various chemicals in a mixture. A very small amount of liquid mixture (i.e., sample) is injected into the instrument and is vaporized in a hot chamber. The sample gas is then pushed by a stream of inert carrier gas through a chromatographic column. This causes various chemicals in the sample gas to take different amounts of time to pass through the column and so appear as sequential concentrations in the stream of carrier gas flowing out of the column. A detector is coupled to the output of the column to detect the existence of these chemicals in the column effluent and their amounts. The term “column effluent” as used herein means a portion of the sample gas that is being eluted out of a chromatographic column at a given time plus a portion of the carrier gas.
One such detector is commonly referred to as a thermal conductivity detector (TCD). Two types of TCDs are known by those skilled in the art. One is a dual-filament TCD. The other type of TCD is a single-filament TCD. FIG. 1 shows an example of one prior art single-filament TCD 10 that has two cavities 12 and 14 and one filament 18 suspended in the cavity 12. The filament 18 is heated to a preset constant temperature. A switch valve 16 is employed to switch at a predetermined frequency, e.g. 5 Hz, throughout the whole detection process, so that the inert carrier gas from a source 100 and the effluent from a column 19 alternatively flow through the filament 18. This causes the voltage required to keep the filament 18 at a given temperature to vary. The voltage is measured and processed, giving rise to an electrical signal which indicates the different compositions in the sample gas.
However, this prior art detector suffers from a gradual decrease in sensitivity due to chemical corrosion of the filament caused by the sample gas. Although the filament is usually fabricated from corrosion-resistant materials, such as Rhenium (Re), Tungsten (W) or Re—W alloy, and has been chemically passivated to protect against oxygen damage, chemical compounds such as acids and halogenated compounds may still attack the filament causing its sensitivity to decrease dramatically. Thus, after some period of time in operation, the filament has to be replaced, resulting in economic loss and inconvenience of operation.
In view of the above problems, it is desirable to protect the TCD filament from chemical corrosion and to prolong its lifespan.